1. Field of the Invention
This invention relates to the processing of residual petroleum charge stocks and in particular to the visbreaking of such charge stocks in the presence of certain highly aromatic petroleum refinery hydrogen-donor materials.
2. Description of the Prior Art
Visbreaking, or viscosity breaking, is a well-known petroleum refining process in which reduced crudes are pyrolyzed, or cracked, under comparatively mild conditions to provide products having lower viscosities and pour points thus reducing the amounts of less-viscous and more valuable blending oils required to make the residual stocks useful as fuel oils. The visbreaker feedstock usually consists of a mixture of two or more refinery streams derived from sources such as atmospheric residuum, vacuum residuum, furfural-extract, propane deasphalted tar and catalytic cracker bottoms. Most of these feedstock components, except the heavy aromatic oils, behave independently in the visbreaking operation. Consequently, the severity of operation for a mixed feed is limited greatly by the least desirable (highest coke forming) components. In a typical visbreaking process, the crude or resid feed is passed through a heater and heated to about 800.degree. to about 975.degree. F. and at about 50 to about 1000 psig. Light gas-oil may be recycled to lower the temperature of the effluent to within about 500.degree. to about 700.degree. F. Cracked products from the reaction chamber are introduced into a flash distillation unit with the vapor overhead being separated in a fractionating column into a light distillate overhead product, e.g., gasoline and light gas-oil bottoms, and the liquid bottoms being separated in a vacuum fractionating column into heavy gas-oil distillate and residual tar. Examples of such visbreaking methods are described in Beuther et al, "Thermal Visbreaking of Heavy Residues," The Oil and Gas Journal, 57:46, Nov. 9, 1959, pp. 151-157; Rhoe et al, "Visbreaking: A Flexible Process," Hydrocarbon Processing, January 1979, pp. 131-136; and U.S. Pat. 4,233,138, all of which are incorporated herein by reference.
Various visbreaking processes are known where residual oils are added to the visbreaking stage with or without added hydrogen or hydrogen-donors. U.S. Pat. No. 3,691,058 discloses production of single ring aromatic hydrocarbon (160.degree.-430.degree. F.) by hydrocracking a heavy hydrocarbon feed (1050.degree. F.-) and recycling 90.degree.-160.degree. F. and 430.degree. F.+ product fractions to extinction. This is integrated with visbreaking of residua in the presence of 1-28 wt. % free radical acceptor at 700.degree.-900.degree. F. in the presence or absence of hydrogen (to enhance residua depolymerization). U.S. Pat. No. 4,067,757 discloses a process comprising of passing a resid up a bed of inert solids (packed bed reactor) in the presence or absence of 50-10,000 SOFH hydrogen at 750.degree.-1000.degree. F. to enhance middle distillate (350.degree.-650.degree. C.) production.
U.S. Pat. No. 2,953,513, incorporated herein by reference, discloses production of hydrogen-donors by partial hydrogenation of certain distillate thermal and catalytic tars, boiling above 700.degree. F., containing a minimum of 40 wt.% aromatics, to contain H/C ratios of 0.7-1.6. The resid feed is then mixed with 9-83 vol. % of hydrogen-donor and thermallycracked at 800.degree.-900.degree. F. to produce low boiling products. U.S. Pat. No. 4,090,947 describes a thermal cracking process (800.degree.-1000.degree.F.) for converting resids to lighter products in the presence of 10-500 vol. % hydrogen-donor. The hydrogen-donor is produced by hydrotreating premium coker gas oil (650.degree.-900.degree. F.) by itself or a blend with gas oil produced in the thermal cracker. U.S. Pat. No. 4,292,168 discloses upgrading heavy hydrocarbon oils without substantial formation of char by heating the oil with hydrogen and a hydrogen transfer solvent without a catalyst at temperatures of about 320.degree.-500.degree. C. (666.degree.-1026.degree. F.) and a pressure of 22-180 bars for a time of about 3-30 minutes. Examples of hydrogen-donor transfer solvents include pyrene, fluoranthene, anthracene, benzanthracene, etc. U.S. Pat. No. 4,292,686 discloses a process for contacting a resid with a hydrogen-donor at 350.degree.-500.degree. C. and a pressure of 2-7 MPa with liquidly hourly space velocities ranging from 0.5-10.
U.S. patent application Ser. No. 519,625 filed Aug. 1, 1983, by Choi, Gross and Malladi which is incorporated herein by reference is directed to an improved process for the production of fuel oil products in which the formation of coke or filtration sediment is suppressed by visbreaking heavy petroleum redidua under liquid phase, non-catalytic conditions in the presence of certain hydrogen-donor materials and in the absence of added free hydrogen. By means of the invention described in that application, heavy petroleum oil feed stocks containing deleterious contaminants such as sulfur and nitrogen compounds, asphaltenes, metals, and the like, can be visbroken at high severities to provide lower molecular weight fuel oil products of improved viscosity and pour point characteristics. The process of that invention further offers the potential of substantially eliminating and/or reducing the need for cutter stock to meet fuel oil product viscosity specifications.